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THE   CATEGORIES   OF  VARIATION 


PROFESSOR  S.  J.  HOLMES 


[Reprinted  from  THE  AMERICAN  NATURALIST,  Vol.  XLIII.,  No.  509,  May,  1909.] 


[Reprinted  from  THE  AMERICAN  NATURALIST,  Vol.  XL1IJ.,  May,  11)09  ) 


THE  CATEGORIES  OF  VARIATION 

PROFESSOR   S.   J.  HOLMES 

UNIVERSITY  OF  T&iaeassstN  <L/)t-tforNt  A 


IT  is  a  well-established  fact  that  what  are  commonly 
called  variations  include  modifications  of  quite  different 
import  in  relation  to  the  process  of  evolution.  Whether 
or  not  the  variations  that  are  induced  in  the  soma,  either 
by  its  own  activities  or  through  the  influences  of  the  en- 
vironment, have  any  effect  in  shaping  the  course  of  evolu- 
tion as  they  were  held  to  do  by  Lamarck  and  his  followers, 
it  is  evident  that  they  do  not  count  in  this  process  in  the 
same  way  as  variations  that  arise  in  the  germ.  But 
among  the  germinal  variations  themselves  there  are 
classes  of  unequal  significance.  Variations  differ  mark- 
edly in  regard  to  their  stability  or  permanence.  Many 
variations  after  their  first  appearance  persist  with  little 
modification  for  an  apparently  indefinite  time.,  Of  these. 
what  are  commonly  called  mutations  afford  conspicuous^ 
examples;  these  are  abrupt  variations  which  breed  true. 
or  nearly  so  from  the  start,  having  their  own  fluctuating 
variability,  to  be  sure,  but  around  a  mean  which  does  not 
approach  that  of  the  parental  type  in  successive  genera- 
tions. Other  variations  behave  quite  differently.  They 
may  be  selected  generation  after  generation,  modifying 
the  stock  up  to  a  certain  point,  after  which,  if  the  variety 
is  left  to  itself,  there  is  revision  towards  the  original  par- 
ent. It  is  held  by  many  "Chat  these  two  classes  of  varia- 
tions are  fundamentally  distinct,  and  that  only  the  first, 

257 


258  THE  AMERICAN  NATURALIST          [VOL.  XLIII 

so-called  discontinuous  variations,  play  an  important  role 
in  the  origination  of  new  species. 

A  considerable  proportion  of  what  is  described  as 
fluctuating  variability  is,  in  many  cases,  simply  somatic 
variation,  having  no  relation  to  the  germ  plasm.  It  is 
evident,  however,  that  all  fluctuating  variability  can  not 
be  such,  otherwise  species  could  not  be  modified  by  ordi- 
nary methods  of  continued  selection.  Our  mathematical 
curves  represent  two  kinds  of  variability  lumped  together 
and  which  it  is  in  most  cases  practically  impossible  to 
separate.  The  character  of  height,  for  instance,  in 
human  beings  is  to  a  certain  extent  an  inherited  one,  but 
it  is  determined  to  a  marked  degree  by  influences  opera- 
ting after  birth.  The  usual  curves  of  variation  represent 
both  and  may  even  include  also  variations  in  the  nature 
of  mutations  which  fail  of  discrimination  from  the  rest 
of  the  aggregate. 

De  Vries  distinguished  three  kinds  of  germinal  varia- 
tions, elementary  species,  retrograde  varieties  and  fluctu- 
ations. These  three  kinds  he  conceives  to  be  sharply 
distinguished  and  produced  in  different  ways.  /All  con- 
genital variability  is  regarded  by  him  as  resting  upon 
qualitative  or  quantitative  changes  in  the  pangens  or  the 
organic  units  of  which  he  conceives  living  matter  to  be 
built  up^The  pangens  form  the  basis  of  the  unit  char- 
acters, or  independently  variable  elements  of  the  organ- 
ism, there  being  a  special  kind  of  pangen  for  each  such 
character.  Variations  in  the  number  of  pangens  cause 
variations  of  the  fluctuating  type  which  obey  Quetelet's 
law  of  chance  frequency  distribution.  I  De  Vries  main- 
tains and  attempts  to  prove  by  the  citation  of  several 
examples  that  through  the  selection  of  such  variations 
modification  may  be  carried  to  a  certain  point,  but  soon 
a  limit  is  reached  beyond  which  selection  is  incompetent 
to  effect  further  improvement.  Moreover,  continued 
selection  must  be  practised  in  order  to  maintain  the  con- 
dition which  has  been  reached,  else  the  stock  will  in  the 
course  of  a  few  generations  revert  more  or  less  completely 
to  the  ancestral  mean.  I 


No.  509]  THE  CATEGORIES  OF  VARIATION  259 

Retrograde  varieties,  according  to  De  Vries,  are 
sharply  distinguished  from  fhictuationst  They  are,  as  a 
rule,  constant  from  the  start,  and  differ  from  the  type 
in  only  one  or  at  most  a  very  few  respects. 

They  originate  for  the  greater  part  in  a  negative  way  by  the  ap- 
parent loss  of  some  quality  and  rarely  in  a  positive  manner  by  acquir- 
ing a  character  seen  in  allied  species."  "  By  far  the  greatest  part 
of  the  ordinary  garden-varieties  differ  from  their  species  by  a  single 
sharp  character  only.  In  derivative  cases,  three  or  even  more  such 
characters  may  be  combined  in  one  variety,  for  instance,  a  dwarfed  - 
variety  of  the  larkspur  may  at  the  same  time  bear  white  flowers  or  even 
double  white  flowers,  but  the  individuality  of  the  single  characters  is 
not  in  the  least  obscured  by  such  combinations. 

These  varieties,  says  De  Vries,  "do  not  possess  any- 
thing really  new."  The  loss  of  a  character  is  merely 
apparent.  "On  a  closer  inquiry  we  are  led  to  the  as- 
sumption of  a  latent  or  dormant  state.  The  presumably 
lost  characters  have  not  absolutely,  or  at  least  not  per- 
manently disappeared.  They  show  their  presence  by 
some  slight  indication  of  the  quality  they  represent,  or 
by  occasional  reversions.  They  are ..„ ilot  wanting,  but 
only  latent. ' '  In  other  words^tj^eTonly  difference  be-  ' 
tween  retrograde  varieties  an^Rhe  types  i§  the  latency 
or  patency  of  certain  charJicters.  The  same  kinds  of 
pangens  are  present  in  the^erm  plasm  of  both. 

Elementary  species,  on  the  other  hand, 

are  distinguished  from  their  nearest  allies  in  almost  all  organs.  There 
is  no  prominent  distinctive  feature  between  the  single  forms  of  Drab  a 
verna,  Helianthemum  or  of  Taraxacum;  all  characters  are  almost 
equally  concerned.  The  elementary  species  of  Draba  are  characterized, 
as  we  have  seen,  by  the  forms  and  the  hairiness  of  the  leaves,  the 
number  and  height  of  the  flower  stalks,  the  breadth  and  incision  of  the 
petals,  the  forms  of  the  fruits,  and  so  on.  Every  one  of  the  two 
hundred  forms  included  in  this  collective  species  has  its  own  type, 
which  it  is  impossible  to  express  by  a  single  term.  Their  names  are 
chosen  arbitrarily.  Quite  the  contrary  is  the  case  with  most  of  the 
varieties,  for  which  one  word  ordinarily  suffices  to  express  the  whole 
difference. 

The  most  important  distinction  which  De  Vries  draws 
between  retrograde  varieties  and  elementary  species  is 
a  physiological  one.  They 


260  THE  AMERICAN  NATURALIST          [VOL.  XLIII 

behave  in  quite  different  manner,  when  subjected  to  crossing  experi- 
ments, and  the  hope  is  justified  that  some  day  crosses  may  become 
the  means  of  deciding,  in  any  given  instance,  what  is  to  be  called 
species,  and  what  variety  on  physiological  grounds. 

When  varieties  are  crossed  with  the  parent  type  the 
character  that  is  active  in  one  or  the  other  of  the  forms 
will  usually  be  patent  in  the  first  generation  of  offspring. 
In  the  second  generation  there  is,  according  to  De  Vries, 
a  segregation  of  characters  which  takes  place  in  con^ 
formity  to  Mendel's  law.  Ordinary  sugar  corn,  for  ex- 
ample, differs  from  the  usual  type  in  having  a  part  of 
the  starch  replaced  by  sugar  in  the  kernels,  which  conse- 
quently become  wrinkled  when  dry.  When  these  two 
forms  are  crossed  the  active  character  of  starchy  kernels 
is  present  in  all  the  members  of  the  first  generation.  In 
the  second  generation  there  is  a  segregation  of  these 
characters,  one  fourth  of  the  offspring  being  wrinkled 
kernels,  and  three  fourths  smooth  ones.  Approximately 
one  third  of  the  latter  produce  only  smooth  kernels  in 
subsequent  generations,  while  the  other  two  thirds  split 
up  again  in  the  expected  Mendelian  ratio. 

In  the  crossing  of  varieties  it  is  possible,  according  to 
De  Vries,  for  all  the  corresponding  characters  of  the  two 
forms  to  become  paired.  As  the  distinguishing  feature 
of  the  variety  is  the  latency  or  patency  of  one  or  more 
characters,  these  characters  "will  unite  as  well  as  though 
they  were  both  active  or  both  dormant.  For  essentially 
they  are  the  same,  only  differing  in  their  degree  of  ac- 
tivity. From  this  we  can  infer  that,  in  the  crossing  of 
varieties,  no  unpaired  remainder  is  left,  all  units  com- 
bining in  pairs  exactly  as  in  ordinary  fertilization."  As 
the  varieties  differ  only  in  the  dominance  or  latency  of 
certain  characters,  offspring  obtained  through  crossing 
them  differ  only  in  the  same  way.  For  such  unions  De 
Vries  gives  the  name  "bisexual  crosses,"  inasmuch  as 
there  is  "complete  bisexuality,  all  unit  characters  com- 
bining in  pairs. ' ' 

In  the  crossing  of  the  elementary  species  of  (Enothera 
De  Vries  found  that  in  the  first  generation  there  was  a 


No.  509]  THE  CATEGORIES  OF  VARIATION  261 

splitting  up  of  the  progeny  in  various  ratios,  but  that  the 
second  and  subsequent  generations  bred  true  to  type, 
thus  presenting  a  condition  just  the  reverse  of  Mendelian 
inheritance.  For  instance  when  the  mutant  rubrinervis 
was  crossed  with  the  parent  type  Lamarckiana  the  first 
generation  of  hybrids  were  either  rubrinervis  or  La- 
marckianas,  the  proportion  varying  greatly  in  different 
lots.  The  two  kinds  of  hybrids  did  not  split  up  in  the 
second  generation  but  bred  true  to  type.  Similar  results 
were  obtained  by  crossing  several  other  elementary 
species  of  Lamarckiana  but  this  kind  of  behavior  does 
not  seem  to  be  generally  characteristic  of  the  elementary, 
species  of  other  forms. 

In  the  crossing  of  elementary  species  there  is,  accord- 
ing to  De  Vries,  one  unit  character  which  is -not  mated, 
since 

the  differentiating  mark  is  present  in  one  of  the  parents  and  not  in 
the  other.  While  all  other  units  are  paired  in  the  hybrid  it  is  not. 
It  meets  with  no  mate  and  must  therefore  remain  unpaired.  The 
hybrid  of  two  such  elementary  species  is  in  some  way  incomplete  and 
unnatural.  In  the  ordinary  course  of  things  all  individuals  derive 
their  qualities  from  both  parents;  for  each  single  mark  they  possess 
at  least  two  units.  Practically  but  not  absolutely  equal,  these  two 
opponents  always  work  together  and  give  to  the  offspring  a  likeness 
to  both  parents.  No  unpaired  qualities  occur  in  normal  offspring; 
these  constitute  the  essential  features  of  the  hybrids  of  species  and 
are  at  the  same  time  the  cause  of  their  wide  deviations  from  ordinary 
rules. 

These  differences  between  variations  were  predicted 
by  De  Vries  on  the  basis  of  his  pangen  theory,  and  in  his 
essay  on  ' i  Intracellular  Pangenesis"  published  in  1889 
he  expresses  the  opinion  that  fluctuating  variability  which 
rests  upon  numerical  variation  of  the  pangens  plays  but 
a  minor  part  in  the  modification  of  species.  The  "art- 
bildende"  or  species  forming  variability,  is  dependent 
upon  the  appearance  of  an  entirely  new  kind  of  pangen. 
When  categories  of  variation  are  anticipated  a  priori  on 
the  basis  of  a  theory  of  the  constitution  of  living  matter 
there  is  naturally  produced  a  temptation  or  bias  towards 


262  THE  AMERICAN  NATURALIST          [VOL.  XLIII 

reading  the  classification  into  nature  and  to  the  over- 
looking of  transitional  stages,  and  we  shall  therefore  en- 
quire if  the  distinction  which  is  made  between  elementary 
species  and  varieties  is  a  valid  one. 
\  In  the  first  place,  there  does  not  seem  to  be  any  very 
good  reason  why  on  the  pangen  theory  elementary  species 
should  differ  in  numerous  characters  from  the  parent 
form.  A  pangen  is  the  basis  of  a  single  unit  character. 
Elementary  species  are  produced  through  the  origination 
of  a  new  kind  of  pangen.  /  If  the  becoming  latent  or 
dominant  of  a  pangen  affects  only  one  unit  character  of 
an  organism,  it  is  not  evident,  when  a  new  kind  of  pangen 
is  produced,  why  the  whole  organization  of  the  plant 
should  be  so  profoundly  influenced.  Why  should  not  the 
awakening  of  a  dormant  pangen  produce  as  great  a 
change  as  the  production  of  a  new  pangen  of  a  somewhat 
different  quality.  Says  De  Vries : 

There  can  be  little  doubt  but  that  all  the  attributes  of  every  new 
species  are  derived  from  one  principal  change.  But  why  this  should 
affect  the  foliage  in  one  manner,  the  flowers  in  another  and  the  fruits 
in  a  third  direction,  remains  obscure.  To  gain  ever  so  little  insight 
into  the  nature  of  these  changes,  we  may  best  compare  the  differences 
of  our  evening  primroses  with  those  between  the  two  hundred  ele- 
mentary species  of  Draba  and  other  similar  instances.  In  doing  so 
we  find  the  same  main  feature:  the  minute  differences  in  nearly  all 
points. 

De  Vries  nowhere  gives  us  a  much  clearer  explanation 
as  to  why  elementary  species  and  varieties  should  differ 
in  this  way  and  we  must  probably  be  content  with  re- 
ferring the  matter  to  different  degrees  of  "correlation." 

It  is  evident  that  there  are  allied  groups  separated  by 
small  differences  throughout  the  entire  organization,  and 
there  are  other  groups  which  differ  apparently  in  single 
characters  only,  such  as  the  presence  or  absence  of  hair, 
spines  or  certain  colors.  Hornless  cattle  and  six-toed 
cats  do  not  seem  to  present  any  general  or  constitutional 
differences  from  the  other  members  of  their  species,  but 
this  is  a  subject  upon  which  we  should  exercise  great 
caution,  as  very  slight  differences  in  the  rest  of  the  organ- 


No.  509]  THE  CATEGORIES  OF  VARIATION  263 

ization  may  be  correlated  with  pronounced  differences 
in  a  single  part. 

Among  the  forms  arising  by  mutation  from  (Enofhera 
lamarckiana  De  Vries  distinguishes  three  varieties, 
lavifolia,  brevistylis  and  nanella.  These  forms  are  called 
varieties  instead  of  elementary  species  because  they  dif- 
fer from  the  type  in  a  few  characters  only  and  because 
of  their  different  behavior  when  crossed.  But  even  ac- 
cording to  De  Vries '  own  description  the  points  of  differ- 
ence are  not  limited  to  a  single  character.  Lcevifolia,  for 
instance,  is 

chiefly  distinguished  from  Lamarck's  evening  primrose  by  its  smooth 
leaves,  as  its  name  indicates.  The  leaves  of  the  original  form  show 
numerous  sinuosities  in  the  blades,  not  at  the  edge,  but  anywhere  be- 
tween the  veins.  The  blade  shows  numbers  of  convexities  on  either 
surface,  the  whole  under  surface  being  undulated  in  this  manner.  It 
lacks  the  brightness  of  the  ordinary  evening-primrose  or  (Enothera 
biennis.  These  undulations  are  lacking  or  at  least  very  rare  on  the 
leaves  of  the  new  Icevi folia.  Ordinarily  they  are  wholly  wanting,  but 
at  times  single  leaves  with  slight  manifestations  of  this  character  may 
make  their  appearance.  They  warn  us  that  the  capacity  for  such 
sinuosities  is  not  wholly  lost,  but  only  lies  dormant  in  the  new  variety. 

The  leaves  of  Icevifolia  are  also  ' '  a  little  narrower  and 
more  slender  than  those  of  the  Lamarckiana."  But 
Icevifolia  also  shows  differences  in  the  flower.  "The 
yellow  color  is  paler  and  the  petals  are  smoother.  Later 
in  the  fall,  on  the  weaker  side  branches  these  differences 
increase.  The  Icevifolia  petals  become  smaller  and  are 
devoid  of  the  emargination  at  the  apex,  becoming  ovate 
instead  of  obcordate." 

Brevistylis  is  characterized  by  its  short  style.  The 
stigma  is  different  in  shape  from  that  of  the  parent  form ; 
there  are  differences  in  the  ovaries,  and  there  are  only 
a  few  seeds  produced.  These  differences  maj  possibly 
depend  upon  a  single  varying  character,  although 

the  leaves  of  the  0.  brevistylis  are  more  rounded  at  the  tip,  but  the 
difference  is  only  pronounced  at  times  slightly  in  the  adult  rosettes;  but 
more  clearly  on  the  growing  summit  of  the  stems  and  bitches.  By 
this  character  the  plants  may  be  discerned  among  the  others,  some 
weeks  before  the  flowers  begin  to  show  themselves. 


264  THE  AMERICAN  NATURALIST          [VOL.  XLIII 

Nanella  is  a  dwarf  plant,  but  it  is  not  distinguished  by 
its  smaller  size  alone. 

From  its  first  leaves  to  the  rosette  period,  and  through  this  to  thd 
lengthening  stem,  the  dwarfs  are  easily  distinguished  from  any  other 
of  its  congeners.  The  most  remarkable  feature  is  the  shape  of  the 
leaves.  They  are  broader  and  shorter,  and  especially  at  the  base  they 
are  broadened  in  such  a  way  as  to  become  apparently  sessile.  The 
stalk  is  very  brittle,  and  any  rough  treatment  may  cause  the  leaves  to 
break  off.  ...  The  stems  are  often  quite  unbranched,  or  branched 
only  at  the  base  of  the  spike.  Strong  secondary  stems  are  a  striking 
attribute  of  the  Lamarckiana  parent,  but  they  are  lacking,  or  almost 
so  in  the  dwarfs. 

So  far  as  morphological  evidence  is  concerned,  the  dif- 
ference between  the  above  forms  and  elementary  species 
are  not  so  sharp  as  to  inspire  one  with  much  confidence 
in  the  essential  distinctiveness  of  the  two  classes.  All 
of  these  so-called  varieties  differ  in  various  parts  of  their 
organization.  It  may  be  said  that  these  differences  are 
dependent  through  correlation  upon  the  variation  of  a 
single  character,  but  if  any  one  maintains  that  smooth 
leaves  and  paler  flowers,  or  small  size,  brittle  stem  and 
short  leaf  stalks  are  related  in  this  way,  the  burden  of 
proof  is  on  his  shoulders.  If  a  half  dozen  characters  in 
different  parts  of  the  plant  vary  it  would  indeed  be  diffi- 
cult, amid  a  considerable  amount  of  fluctuating  varia- 
bility, to  separate  on  morphological  grounds  a  retrograde 
variety  from  a  true  elementary  species,  especially  since 
experts  are  sometimes  troubled  in  distinguishing  some 
of  the  elementary  species  from  one  another.  Indeed,  De 
Vries  admits  that  it  is  often  very  difficult  to  decide 
whether  a  given  form  belongs  to  one  or  the  other  of  these/ 
two  groups,  but  he  states  that  in  such  cases  we  have  a 
means  of  testing  the  matter  experimentally  by  the  forma- 
tion of  crosses.  Let  us  see,  therefore,  how  the  test  of 
crossing  works  out. 

In  the  case  of  the  varieties  of  (Enothera  lamarckiana 
there  is  in  the  second  generation  a  splitting  according  to 
the  Mendelian  ratio  when  the  variety  is  crossed  with  the 
parent  form,  but  with  varieties  of  other  forms  this  does 


No.  509]     •        THE  CATEGORIES  OF  VARIATION  265 

not  seem  to  be  an  invariable  occurrence.  Davenport  has 
shown  that  albinism  in  poultry  is  in  some  cases  a  non- 
Mendelian  character,  and  the  same  is  probably  true,  ac- 
cording to  Castle,  for  the  lop-eared  condition  in  rabbits. 
In  the  inheritance  of  long  and  short  hair  in  guinea  pigs 
there  is  also  a  marked  departure  from  Mendelian  ratios. 
In  silkworms  Kellogg  has  shown  that  while  most  features 
are  Menaelian,  cocoon  color  in  some  cases  follows  Men- 
delian ratios,  but  in  others  it  proved  to  be  "  inconstant 
as  to  dominance  and  recessiveness  and  numerical  propor- 
tions, and  may  even  break  down  and  blend."  Deaf- 
mutism  also  refuses  to  come  under  Mendelian  categories 
according  to  the  statistics  compiled  by  Bell.  The  fore- 
going are  cases  of  apparent  retrograde  variations  which 
form  an  exception  to  Mendel's  law,  but  it  must  be  ad- 
mitted that  the  majority  of  such  variations  which  have 
been  investigated  show  a  fair  approximation  to  Men- 
delian ratios. 

In  the  crosses  between  the  elementary  species  of 
(Enothera  lamarckiana  there  is  commonly  a  splitting 
up  in  the  first  generation  with  absence  of  splitting  in  the 
second  and  subsequent  ones.  Hybrids  of  0.  Lamarckiana 
and  0.  biennis,  however,  have  nearly  the  aspect  of  the 
latter  species  and  remain  true  in  the  second  and  subse- 
quent generations  without  reversion  or  splitting.  Crosses 
between  0.  muricata  and  0.  Mrtella  produced  hybrids 
showing  the  characters  of  both  parents.  These  were 
propagated  through  four  generations  and  remained  "true 
to  /this  type,  showing  <&nly  slight  fluctuations  and  never 
reverting  or  segregating  the  mixed  characters."  In- 
stances of  constant  hybrids  between  different  species  are 
very  common  and  it  is  unnecessary  to  specify  them  here. 
Such  constancy  according  to  De  Vries  is  ' i  one  of  the  best 
proofs  of  unisexual  unions"  or  unions  between  distinct 
elementary  species  in  which  there  is  always  one  unpaired 
pangen. 

The  attempt  to  make  a  general  rule  for  the  hybridizing 
of  elementary  species  leads  to  many  difficulties.  In 


266  THE  AMERICAN  NATURALIST  [VOL.  XLIII 

poultry,  as  Davenport  has  pointed  out,  such  characters 
as  pea  and  rose  comb,  extra  toes  and  the  presence  of 
muffs  and  beards  on  the  head,  are  acquisitions  which 
developed  since  the  domestication  of  the  original  an- 
cestral species.  They  certainly  can  not  be  regarded 
as  the  outcropping  of  latent  characters  which  are  rep- 
resented in  allied  forms,  but  are  in  the  line  of  progres- 
sive  variation  and  therefore  according  to<rtKebry"  de- 

ire 

Mendel's  law,  the  extra  toes  do  not  seem  to  come  under 
-any  definite  rule  of  inheritance,  and  none  of  them  follow 
the  rule  for  the  hybridization  of  elementary  species. 

Consider  the  forms  of  the  common  potato  beetle  studied 
by  Tower.  These  arise  suddenly,  breed  true  to  type  and 
differ  from  the  parent  form  in  many  characters,  some  of 
which  are  apparently  in  the  line  of  progressive  evolution. 
They  seem  to  be  as  truly  elementary  species  as  the 
mutants  of  (Enothera  lamarckiana.  Yet  when  crossed 
with  the  parental  type  they  produce  hybrids  which  in 
most  cases  give  a  mixed  progeny  segregating  according 
to  Mendel's  law.  If  we  can  not  call  these  forms  ele- 
mentary species  there  is  no  way  of  distinguishing  such 
except  through  breeding  experiments,  and  the  distinction 
De  Vries  draws  between  elementary  species  and  varieties 
amounts  to  nothing  more  than  the  fact  that  crosses  be- 
tween certain  groups  follow  Mendel's  law,  while  crosses 
between  others  do  not.  There  is  no  correlation  between 
any  structural  criterion  of  species  and  the  criterion 
afforded  by  breeding  experiments. 

Now  when  we  attempt  to  make  a  classification  on  the 
basis  of  breeding  experiments  alone  we  fare  little  better. 
/  With  blended  inheritance  in  the  first  and  all  subsequent 
generations,  partially  blended  inheritance,  total  resem- 
blance of  hybrid  to  one  or  another  parent  with  or  without 
subsequent  splitting,  incomplete  segregation  of  char- 
acters, splitting  of  offspring  of  hybrids  in  various  incon- 
stant and  non-Mendelian  ratios,  and  many  other  irregular 


No.  509]  THE  CATEGORIES  OF  VARIATION  267 

manifestations  of  heredity,  the  difficulty  of  maintaining 
a  sharp  distinction  between  varieties  and  elementary 
species  on  the  basis  of  behavior  in  inheritance  is  apparent. 
Are  we  to  classify  a  six-toed  cat  as  a  variety  or  an  ele- 
mentary species?  The  variation  is  apparently  limited 
to  a  single  character  and  it  has  therefore  one  of  the  marks 
of  a  variety,  but  the  variation  is  doubtless  a  progressive 
one  and  not  due  to  an  awakening  of  a  latent  character,  and 
hence  possesses  one  of  the  features  of  an  elementary 
species.  When  crossed  some  of  the  offspring  of  the  • 
first  generation  may  inherit  the  variation  and  some  not, 
and  the  same  is  true  for  the  following  generation;  but 
there  is  apparently  no  splitting  according  to  the  law  of 
Mendel.  So  far  as  our  knowledge  goes  the  situation  is 
the  same  in  respect  to  polydactylism  in  man. 

The  second  volume  of  the  Mutationstheorie,  which 
seems  to  have  been  little  read  by  most  expositors  of  De 
Vries,  affords  several  examples  of  irregular  behavior  of 
the  hybrids  between  elementary  species  which  are  very 
difficult  to  classify.  Crosses  between  (Enothera  nanella 
and  0.  rubrinervis,  for  instance,  the  one  *a  retrograde 
variety  of  Lamarckiana  and  the  other  a  distinct  elemen- 
tary species,  gave  very  variable  results,  with  splittings 
in  the  first  and  succeeding  generations  in  very  inconstant 
ratios,  and  the  occasional  production  of  blends  which 
bred  fairly  true.  We  have  here  a  curious  combination 
of  the  characteristics  of  unisexual  and  bisexual  crosses. 

It  would  not  be  difficult  to  bring  forward  many  other  j 
cases  which  refuse  to  fall  within  the  scheme  of  classifica- 
tion propounded  by  De  Vries.  There  are  many  kinds 
of  variations  which  are  inherited  in  many  kinds  of  ways. 
The  pangen  theory  of  the  celebrated  botanist  has  proved 
a  deceptive  guide  and  has  led  its  author  to  do  scant  justice 
to  many  classes  of  facts  which  do  not  fall  in  line  with  it. 
Hypotheses  about  paired  and  unpaired  pangens  have  de- 
termined De  Vries 's  classification  of  the  different  kinds 
of  variations  and  profoundly  influenced  his  interpretation 
of  his  extensive  and  valuable  researches.  The  doctrine 


268  THE  AMERICAN  NATURALIST          [VOL.  XLIII 

of  intracellular  pangenesis  has  never  received  the  logical 
development  that  characterizes  Weismann's  theory  of 
the  germ  plasm  and  is  considerably  inferior  to  the  latter 
as  a  scholastic  production.  The  explanation  it  affords 
of  the  alleged  distinction  between  varieties  and  elemen- 
tary species  is,  as  we  have  seen,  practically  no  explanation 
at  all.  The  theory  may  be  consistent  with  the  facts  of 
Mendelian  inheritance  and  the  supposed  independent 
variability  of  parts,  but  why  it  should  lead  one  to  antici- 
pate that  elementary  species  differing  from  the  parent 
throughout  their  organization  originate  by  a  single  sud- 
den leap  is  not  so  clear.  Rather  it  would  lead  one  to 
expect  that  organisms  would  be  modified,  a  part  here  and 
a  part  there,  corresponding  to  the  independently  variable 
elements  determined  by  particular  pangens,  of  which  there 
are  numerous  kinds,  until  the  whole  was  slowly  trans- 
formed. De  Vries,  however,  is  careful  to  explain  that  a 
single  pangen  may  be  responsible  for  certain  characters 
found  in  various  parts  of  the  organism,  such  as  the  color 
of  leaves,  flower  and  fruit,  and  that  pangens  are  supposed 
to  influence  each  other's  manifestation  so  that  a  variation 
in  a  single  pangen  may  have  a  far-reaching  effect.  In 
a  chapter  on  the  association  of  characters  in  his  recent 
book  on  Plant  Breeding  a  great  deal  of  emphasis  is  laid 
upon  the  value  of  a  study  of  the  correlation  of  the  dif- 
ferent parts  of  the  plant.  He  says : 

We  come  to  the  conception  of  a  general  interdependency  of  all 
parts,  organs  and  qualities  of  an  organism.  They  are  governed  more 
or  less  by  the  same  laws  which  cause  them  to  undergo  corresponding 
changes  when  subjected  to  the  same  influences. 

It  seems  to  me  that  the  author  is  here  upon  treacherous 
ground.  Through  the  assumption  of  manifold  correla- 
tions De  Vries  attempts  to  account  for  a  change  in  a 
single  pangen  which  has  to  do  primarily  with  one  inde- 
pendently variable  part  of  the  organism  producing  a 
modification  of  the  organism  as  a  whole,  but  in  so  doing 
he  is  taking  the  foundation  away  from  the  argument  upon 
which  the  justification  of  the  pangen  assumption  rests. 


No.  509]  THE  CATEGORIES  OF  VARIATION  269 

To  the  extent  that  the  organism  is  a  whole  of  interde- 
pendent parts,  to  just  that  extent  it  gives  evidence  of  not 
being  a  piece  of  mosaic  work  and  hence  removes  the  neces- 
sity for  a  hypothesis  of  discrete  germinal  units.  De 
Vries  even  goes  so  far  as  to  say  that  "in  order  to  be 
correlated  the  characters  must  begin  by  being  independ- 
ent entities  which  through  some  later  means  may  come 
into  relation  with  others. ' '  At  one  time  it  is  argued  that 
the  existence  of  pangens  is  proved  by  the  fact  that  the 
parts  of  the  organism  are  independently  variable;  and 
now  it  is  said  that  we  must  assume  that  pangens  must 
exist  to  account  for  the  parts  being  correlated;  that  is, 
for  the  fact  that  they  are  not  independently  variable ! 

The  most  salient  feature  of  the  mutation  theory  is  that 
the  process  of  evolution  is  conceived  to  take  place  by 
sudden  steps  of  considerable  magnitude.  This  has  been 
heralded  with  eclat  as  enabling  us  to  get  rid  of  certain 
difficulties  inherent  in  the  Darwinian  theory,  such  as  the 
assumed  absence  of  intermediate  forms  between  existing 
and  fossil  species  and  even  the  contention  that  the  geo- 
logical history  does  not  afford  time  enough  for  the  process 
of  evolution  as  it  was  formerly  conceived  to  take  place. 
But  it  is  not  correct  to  say  that  a  mutation  is  neces- 
sarily a  large  variation.  Mutations  may  be  very  small 
steps,  falling  far  within  the  limits  of  ordinary  fluctuating 
variability,  as  is  especially  emphasized  by  De  Vries  in 
his  later  writings. 

In  groups  (such  as  brambles,  roses,  buttercups,  willows  and  many 
others)  where  large  numbers  of  species  are  closely  allied,  the  differ- 
ences between  any  two  of  them  become  smaller,  and  the  number  of 
distinct  forms  increasing,  the  distinction  in  the  end  may  become  re- 
duced to  a  single  differential  mark  for  each  two  neighboring  types. 
Such  differences  must  be  assumed  to  be  produced  each  by  a  single 
mutation. 

In  the  light  of  experiments  made  at  Svalof,  De  Vries 
now  concludes  that  "ordinary  varieties  of  cereals  are 
built  up  of  hundreds  of  elementary  forms  which  with  few 
exceptions  have  hitherto  escaped  observation.  The  high 
variability  which  is  commonly  attributed  to  our  ordinary 


270  THE  AMERICAN  NATURALIST          [VOL.  XLIU 

varieties  of  cereals  consists  only  in  the  differences  among 
the  constituents  of  the  mixtures. ' '  Much  of  the  improve- 
ment of  grains  that  was  formerly  obtained  by  continued 
selection  De  Vries  ascribes  to  the  unconscious  selection 
of  elementary  species  and  the  gradual  improvement  of  an 
originally  mixed  stock.  Kimpau's  rye,  a  stable  race 
obtained  by  gradual  selection,  is  thus  accounted  for,  but 
the  burden  of  proof  is  here  on  the  part  of  the  mutationists. 
It  is  apparently  not  so  easy  to  test  the  role  of  mutations 
versus  fluctuations  in  the  improvement  of  species,  because 
if  one  should  secure  a  stable  race  by  the  usual  process 
of  selection,  the  mutationist  might  urge  that,  after  all, 
amid  the  confusion  of  seemingly  fluctuating  variability, 
there  were  some  mutations  which  escaped  notice,  and, 
through  the  unconscious  selection  of  these  and  their  off- 
spring, the  stock  was  gradually  purified  and  converted 
into  an  improved  stable  form. 

Where  ordinary  varieties  include  "hundreds  of  ele- 
mentary forms,"  separated  by  characters  which  in  many 
cases  are  so  small  that  they  *  *  may  be  scarcely  perceptible 
to  the  inexperienced  eye,"  how  is  one  to  tell  whether  he 
is  dealing  with  mutations  or  ordinary  fluctuations  ?  The 
latter  may  be  much  greater  in  extent,  and,  as  we  have  seen, 

\J  there  is  no  structural  criterion  by  which  a  mutation  may 
be  recognized.  Crossing  experiments  give  us  no  certain 
test  and  we  have  therefore  to  fall  back  upon  the  criterion 
of  stability  and  class  as  mutations  those  variations  which 

y  /breed  true  from  their  first  appearance.  Here  the  oppor- 
tunities for  begging  the  question  are  excellent.  If  by 
the  ordinary  process  of  selection  a  stable  race  is  pro- 
duced we  can  of  course  ascribe  it  to  the  unconscious  choice 
of  one  or  more  undetected  mutations.  To  be  sure,  stable 
races  can  be  produced  only  on  the  basis  of  stable  varia- 
tions, and  if  this  is  all  that  the  mutation  theory  neces- 
sarily implies  its  divergence  from  Darwin's  teaching  is 
not  very  wide. 

If  now  it  should  turn  out  that  stability  is  a  matter  of 
degree  the  last  distinguishing  feature  of  the  mutation 


No.  509]  THE  CATEGORIES  OF  VARIATION  271 

theory  would  be  destroyed.  This  is  a  question  upon 
which  we  are  sadly  in  need  of  light.  Some  of  De  Vries  's 
own  mutations  are,  however,  quite  inconstant  and  show 
a  strong  tendency  to  revert  to  the  parent  species. 
(Enothera  scintillans,  when  self -pollinated,  produced  from 
8  per  cent,  to  52.9  per  cent,  of  Lamarckianas  and  34  per 
cent,  to  69  per  cent,  of  its  own  kind  and  a  variable  number 
of  other  mutants.  0.  elliptica  and  0.  linearis  repeat  their 
kind  in  still  smaller  ratios.  The  reversion  of  these 
mutants  like  their  origin  is  sudden,  but  it  shows  an  un- 
stable condition  of  their  germ  plasm  and  it  may  be  ques- 
tioned if  this  reversion  is  essentially  different  from  the 
slower  reversion  which  often  follows  the  cessation  of  the 
selection  of  ordinary  fluctuations. 

That  the  differences  between  mutations  and  fluctua- 
tions are  not  so  fundamental  as  the  pangen  theory  implies 
is  indicated  by  several  facts,  some  of  the  most  suggestive 
of  which  have  been  furnished  by  De  Vries 's  own  experi- 
ments. For  a  number  of  years  De  Vries  carried  on  a 
series  of  experiments  on  the  corn-marigold  Chrysan- 
themum segetum,  with  the  purpose  of  creating  a  double- 
flowered  variety.  De  Vries  chose  a  garden  variety  of 
this  form,  grandiflorum,  and  raised  several  generations, 
selecting  the  seed  each  time  from  heads  which  contained 
1.3  rays  florets.  After  four  years  of  propagation,  when 
he  was  satisfied  of  the  purity  of  the  isolated  strain,  De 
Vries  began  to  discard  all  plants  with  less  than  21  rays 
in  the  terminal  head.  The  selection  was  continued  for 
a  number  of  generations  when  a  plant  appeared  which 
seemed  to  form  a  promising  one  for  the  production  of  a 
double  variety. 

It  was  not  remarkable  for  its  terminal  head,  which  exhibited  the 
average  number  of  rays  of  the  21-rayed  race.  Nor  was  it  distinguished 
by  the  average  figure  for  all  the  heads.  It  was  only  selected  because 
it  was  the  one  plant  which  had  some  secondary  heads  with  one  ray  more 
than  all  the  others.  This  indication  was  very  slight,  and  could  not 
have  been  detected  save  by  the  counting  of  the  rays  of  thousands  of 
heads.  But  the  rarity  of  the  anomaly  was  exactly  the  indication 
wanted,  and  the  same  deviation  would  have  had  no  significance  what- 


272  THE  AMERICAN  NATURALIST          [VOL.  XLIII 

ever  had  it  occurred  in  a  group  fluctuating  symmetrically  around  the 
average  figure.  On  the  other  hand,  the  observed  anomaly  was  only 
an  indication,  and  no  guarantee  of  future  developments. 

From  this  slight  indication  De  Vries  selected  for  three 
years  more  and  found  that  the 

average  number  of  rays  increased  rapidly  and  with  it  the  maximum 
of  the  whole  strain.  The  average  came  up  from  21  to  34.  ...  The 
largest  numbers  determined  in  the  succeeding  generations  increased  by 
leaps  from  21  to  34  in  the  first  year,  and  thence  to  48  and  66  in  the  two 
succeeding  summers. 

Up  to  this  time,  while  there  was  a  great  increase  in  the 
number  of  ray  florets,  there  was  no  trace  of  doubling,  but 
in  a  few  of  the  best  heads  "the  new  character  suddenly 
made  its  appearance. "  If  sudden,  the  step  was  certainly 
a  very  modest  one.  A  single  plant  was  found  in  which 
careful  inspection  revealed  ' '  three  young  heads  with  some 
few  rays  in  the  midst  of  the  disk."  "Had  the  germ  of 
the  mutation, "  asks  De  Vries,  "lain  hidden  through  all 
this  time?  Had  it  been  present,  though  dormant,  in  the 
original  sample  seed  ?  Or  had  an  entirely  new  creation 
taken  place  during  my  continuous  endeavors?  Perhaps 
as  their  more  or  less  immediate  result?"  It  is  stated 
that  "The  new  variety  came  into  existence  at  once"— 
but  when?  While  certain  that  a  mutation  must  have 
appeared,  De  Vries  is  uncertain  when  and  where  it  ap- 
peared. ' '  The  leap, ' '  he  says,  ' i  may  have  been  made  by 
the  ancestor  of  the  year  1895,  or  by  the  plant  of  1899 
which  showed  the  first  central  rays,  or  the  sport  may  have 
been  gradually  built  up  during  these  four  years"  (italics 
mine). 

During  the  next  two  years  improvement  by  selection 
was  kept  up. 

The  average  number  of  rays  which  had  already  risen  from  13  to 
34  now  at  once  came  up  to  47  and  55.  .  .  .  The  maximum  numbers 
came  as  high  as  100  in  1900  and  even  200  in  1901.  .  .  .  Real  atavists 
or  real  reversionists  are  seen  no  more  after  the  first  purification  of 
the  race. 

A  variety  which  is  pronounced  "permanent  and  con- 
stant" was  produced  whose  lower  limit  of  the  number 


No.  509]  THE  CATEGORIES  OF  VARIATION  273 

of  rays  was  raised  to  about  34,  "a  figure  never  reached 
by  the  grandiflora  parent." 

The  unbiased  reader  who  has  carefully  followed  the 
account  of  the  production  of  this  double  flower  can 
scarcely  escape  the  feeling  that  the  interpretation  of  the 
facts  according  to  the  mutation  theory  is  at  times  some- 
what strained.  The  starting  point  of  the  whole  process  is 
the  selection  of  fluctuations.  Now  and  then  the  selection 
of  a  somewhat  more  pronounced  variation  was  made ;  but 
the  so-called  mutations  had  very  small  and  weak  begin- 
nings, and  De  Vries  is  uncertain  just  where  they  occurred 
and  even  suggests  that  they  may  have  been  built  up  grad- 
ually! The  selection  of  fluctuations  seems  to  have  had 
the  effect  of  inducing  variations  of  greater  stability,  if 
not  greater  extent,  in  the  same  direction.  It  is  not  unrea- 
sonable to  suppose  that  the  appearance  of  florets  with 
ligulate  corolla  on  the  disk  is  due  to  the  same  factors 
which  cause  the  increase  in  the  number  of  ray  florets,  and 
the  variation  may  be  in  reality  not  so  discontinuous  as 
it  seems.  In  fact  we  are  ignorant  of  the  stability  or  the 
real  discontinuity  of  many  of  the  steps  in  advance  towards 
the  production  of  the  double  flower.  If  a  mutation  can 
make  its  appearance  as  an  extra  ray  floret  in  one  case, 
and  by  the  occurrence  of  two  or  three  ligulate  corollas 
on  the  disk  of  a  few  flowers  of  the  plant  in  another,  and 
if  both  these  characters  can  be  increased  by  selection  until 
they  reach  a  stable  condition  that  is  far  more  highly 
developed  than  their  original  one,  the  facts  do  not  lend 
much  support  to  a  theory  of  the  saltatory  origin  of 
species.  Rather  they  would  indicate  that  species  have 
been  formed  along  lines  determined  by  selective  processes 
much  in  the  same  way  as  Darwin  conceived  them  to  be. 

By  a  similar  method  of  selection  Burbank  has  produced 
a  scarlet  variety  of  the  California  poppy,  Eschsclioltzia 
calif ornica.  He  noticed  one  flower  with  a  fine  scarlet  line 
on  one  petal.  From  the  seed  of  this  plant  other  poppies 
with  scarlet  lines  were  produced,  but  only  to  a  slight 
extent.  After  selection  was  practised  for  some  years 


274  THE  AMERICAN  NATURALIST  [VOL.  XLIII 

a  race  of  pure  scarlet  poppies  was  finally  obtained  with 
no  indication  of  their  yellow  ancestor.  This  case  is  cited 
by  De  Vries  as  one  of  mutation,  but  certainly  it  required 
more  than  one  mutation  to  bring  about  the  result.1 

Discontinuity  may  often  be  more  apparent  than  real, 
the  discontinuous  variations  in  the  soma  being  the  out- 
come of  continuous  variability  instead  of  abrupt  changes 
in  the  germ.  Let  us  consider  from  this  point  of  view 
the  occurrence  of  digital  anomalies  such  as  polydactylism, 
cleft  hand,  etc.,  which  are  frequently  cited  as  illustrations 
of  discontinuous  variability.  These  anomalies  are  often 
strongly  inherited,  but  in  most  cases  which  have  been 
fully  studied,  the  inheritance,  while  partly  alternative, 
is  not  Mendelian.  In  the  race  of  polydactylous  guinea 
pigs  which  Castle  has  produced  and  bred  for  a  number 
of  generations  the  anomaly  appeared  in  different  indi- 
viduals in  various  stages  of  completeness.  The  parent 
of  the  group,  a  male,  bore  an  imperfectly  developed  toe 
on  his  left  hind  foot.  The  extra  toe  contained  a  claw  and 
probably  the  phalanges,  but  it  was  loosely  attached  and 
hung  limply  down  on  one  side.  This  male  produced  27 
young,  of  which  15  were  polydactylous.  Of  the  latter  some 
had  an  extra  digit  on  both  hind  feet,  others  had  it  on  but 
one,  and  in  a  few  individuals  the  digit  was  more  fully 
developed  than  in  the  father.  In  subsequent  generations 
the  anomaly  appeared  in  very  different  degrees  of  de- 
velopment, some  animals  having  a  fully  developed  digit, 
others  having  a  loosely  hanging  toe  with  or  without  a  nail, 
while  in  extreme  cases  there  was  only  a  fleshy  bag  of  skin 
without  bones  or  claw  which  often  shriveled  up  and  dis- 
appeared a  few  days  after  birth.  The  variation,  when 
appearing  on  one  side  alone  more  frequently  was  limited 
to  the  left  side  (1.  630,  r.  589),  and  when  unequal  on  the 
two  sides  the  left  one  was  usually  the  better  developed. 
Normal  and  polydactylous  individuals  did  not  segregate 
in  Mendelian  ratios.  In  some  instances  the  normal  condi- 

aSee  also  the  experiments  of  MacCurdy  and  Castle  in  relation  of  con- 
tinuous and  discontinuous  variation  in  rats.  Publications  of  the  Carnegie 
Institution,  No.  70,  3907. 


No.  509]  THE  CATEGORIES  OF  VARIATION  275 

tion  gave  evidence  of  being  recessive,  but  this  was  not 
borne  out  by  many  other  cases  in  which  crosses  be- 
tween normal  individuals  produced  polydactylous  young. 
Crosses  of  normal  individuals  both  of  which  were  of 
polydactylous  ancestry  yielded  a  much  higher  per  cent, 
of  polydactylous  young  than  did  crosses  in  which  one 
individual  came  from  a  normal  breed,  thus  showing  a 
certain  tendency  in  the  blood  towards  polydactylism  even 
when  it  did  not  manifest  itself  by  any  outward  mark. 
Different  males  of  the  same  amount  of  polydactylous 
ancestry  often  showed  great  variation  in  the  potency 
with  which  they  were  able  to  transmit  the  anomalous 
character. 

The  evidence  goes  to  show  that  we  are  dealing  here 
with  a  tendency  which,  whatever  may  be  its  basis,  varies 
continuously  and  not  abruptly,  although  producing  varia- 
tions which,  taken  alone,  would  naturally  be  classed  as 
mutations.  The  extra  toe  is  a  new  character,  but  the 
polydactylous  breed  behaves  neither  like  an  elementary 
species  nor  like  a  retrograde  variety.  The  character  fluc- 
tuates to  the  vanishing  point  and  even  beyond  (as  shown 
by  crossing  experiments  with  individuals  of  different  an- 
cestry) and  shows  varying  degrees  of  fidelity  of  trans- 
mission in  different  strains.  Do  we  not  have  a  condition 
intermediate  between  the  abrupt  discontinuous  variations 
which  breed  true  with  great  fidelity  and  ordinary  fluctua- 
tions ?  It  might  be  said  that  we  have  to  do  with  a  muta- 
tion which  fluctuates  to  -an  unusual  degree,  although  it 
originally  depended  upon  a  sudden  change  in  the  germ 
plasm;  but  the  assertion  would  have  no  evidence  to  rest 
upon.  It  might  be  said,  on  the  other  hand,  that  the  varia- 
tion is  dependent  on  the  undue  activity  of  some  of  the 
factors  of  normal  development,  an  expression,  for  in- 
stance of  increased  growth  tendency  in  the  part  at  a  cer- 
tain period,  and  that  this  tendency  is  kept  from  definite 
expression  until  it  reaches  a  certain  strength,  when  it 
manifests  itself  as  a  sudden  variation.  This  conclusion 
is  warranted,  I  believe,  not  only  by  the  great  variability 


276  THE  AMERICAN  NATURALIST          [VOL.  XLIII 

of  the  anomaly,  but  by  the  fact  already  cited  that  normal 
individuals  of  abnormal  ancestry  are  more  apt  to  produce 
abnormal  offspring  than  are  normal  individuals  of  an- 
other strain. 

The  studies  of  Lewis  and  Embleton  and  of  Pearson  on 
the  inheritance  of  split  hand  and  split  foot  in  man  yield 
results  in  many  respects  similar  to  the  preceding.  Al- 
though the  normal  condition  seems  to  be  recessive,  segre- 
gation does  not  occur  in  Mendelian  ratios.  Often  both 
hands  and  both  feet  were  abnormal,  but  frequently  not 
in  the  same  way,  and  in  many  cases  there  were  marked 
differences  in  the  variations  on  the  two  sides  of  the  body. 
As  Pearson  remarks,  it  is  difficult  to  specify  in  such  cases 
what  the  unit  character  may  be.  With  this,  that  or  the 
other  bone  present  in  some  individuals  and  absent  in 
others  and  represented  in  very  varying  degrees  of  de- 
velopment, the  inheritance  gives  little  evidence  of  definite 
units  of  any  kind.  What  is  inherited  appears  to  be  a 
condition  which  manifests  itself  in  varying  ways  and  de- 
grees and  which  can  not  be  accounted  for  by  any  theory 
of  the  sharp  segregation  of  characters. 

Why  certain  germinal  variations  are  strongly  inherited 
and  others  not  is  a  problem  of  much  interest,  but  the  solu- 
tion of  it  may  lie,  not  in  the  supposed  behavior  of  distinct 
morphological  entities  representing  certain  parts,  but  in 
the  physiological  relations  of  the  basis  of  the  variation 
to  the  organized  structure  of  the  germ  plasm.  The  sex 
cells  are  organisms  as  well  as  the  bodies  that  arise  from 
them;  they  have  the  same  capacity  for  self  regulation; 
and  it  is  not  at  all  probable  that  all  kinds  of  variations 
that  may  arise  in  response  to  the  various  influences  to 
which  they  are  subjected  should  be  retained  to  the  same 
degree.  Weismann  has  made  the  suggestive  comparison 
between  the  variations  of  an  organism  and  the  oscillations 
of  a  polyhedron  on  one  of  the  faces  upon  which  it  rests. 
If  the  oscillations  are  small  the  body  tends  to  come  to 
rest  in  the  same  situation  as  before ;  if  they  are  larger  it 
may  topple  over  upon  a  new  face  about  which  it  mav  oscil- 


No.  509]  THE  CATEGORIES  OF  VARIATION  277 

late  as  around  a  new  center  of  equilibrium.  Weismann 
postulates  a  self-regulating  power  in  the  germ  plasm 
which  keeps  numerous  minor  fluctuating  variations  from 
producing  any  essential  modification  of  the  stock.  If, 
however,  a  certain  variation  forms  a  new  center  of  stabil- 
ity it  may  be  permanent.  The  various  mutants  of  (Eno- 
thera  lamarckiana,2  most  if  not  all  of  which  contain  the 
potency  of  giving  rise  to  any  of  the  others  and  which  pre- 
sent very  different  degrees  of  stability,  may  be  due  to 
more  or  less  stable  forms  which  the  germ  plasm  may  as- 
sume rather  than  the  creation  of  new  kinds  of  germinal 
units.  The  stability  of  a  variation  may  be  due,  however, 
not  so  much  to  its  extent  as  the  analogy  with  the  polyhe- 
dron might  lead  us  to  expect  as  to  its  kind.  Variations 
which  are  physiologically  congruent  with  the  organized 
structure  of  the  germ  plasm  form  stable  races;  those 
which  are  not  tend  to  become  reduced  sooner  or  later  to 
the  norm  through  the  regulatory  activity  of  this  sub- 
stance. 

The  germ  plasm  may  be  conceived  to  exercise,  in  regard 
to  its  variations,  a  kind  of  selective  activity  which  may 
manifest  itself  as  a  proneness  of  the  organism  to  vary 
along  certain  lines.  It  is  well  known  that  there  are  partic- 
ular types  of  variation  which  crop  out  independently  and 
mo¥e  or  less  frequently  and  may  be  faithfully  perpetu- 
ated. Polydactylism,  split-hand  and  split-foot,  albinism 
and  melanism,  the  appearance  of  races  of  hairless  animals 
and  glabrous  plants,  the  development  of  nectarines  from 
peaches  and  peaches  from  nectarines,  the  origin  of  peloric 
flowers,  etc.,  have  occurred  many  times  in  independent 

2  It  is  of  course  possible  that  the  mutations  of  (Enothera  Lamarckiana 
result  from  the  impurity  of  the  stock.  The  species  has  been  for  a  long 
time  cultivated  as  an  ornamental  flower,  and  we  have  nothing  but  conjecture 
regarding  its  origin.  Should  it  turn  out  to  be  derived  from  a  mixture  of 
two  or  more  forms  the  mutation  theory  would  be  deprived  of  some  of  its 
best  evidence,  but  there  would  still  remain  a  considerable  number  of  muta- 
tions from  pure  ancestry.  In  (Enothera  gigas  the  interesting  fact  has 
been  discovered  that  there  is  double  the  normal  number  of  chromosomes. 
Whether  this  is  the  cause  or  the  effect  or  merely  one  instance  of  the  dif- 
ferences between  this  mutant  and  the  type  is  unknown. 


278  THE  AMERICAN  NATURALIST          [VOL.  XLIII 

strains.  These  phenomena  may  be  compared  to  various 
anomalies  which  take  their  origin  in  the  soma.  The  em- 
bryo in  its  development  is  liable  to  certain  accidents 
resulting  in  the  production  of  teratologies!  phenomena 
such  as  hare-lip,  double  formations,  anencephaly  and 
many  others.  These  anomalies  fall  into  certain  classes 
and  in  many  cases  can  be  attributed  to  particular  defects 
of  development.  The  germ  plasm  also  may  be  regarded 
as  liable  to  certain  classes  of  accidental  modifications 
which  produce  heritable  variations  of  more  or  less  clearly 
defined  types.  No  one  would  think  of  attributing  anom- 
alies of  somatic  origin  to  the  development  of  a  new  kind 
of  organic  unit.  If  the  same  mutation  appears  time  after 
time,  would  it  not  be  more  reasonable  to  suppose  that  it 
arose  after  the  fashion  of  somatic  anomalies  than  that  it 
depended  upon  the  creation  each  time  of  the  same  kind 
of  a  new  pangen?  The  fact  that  mutations  can  be  in- 
duced through  the  influence  of  the  environment  certainly 
favors  such  a  view.  Tower  found  that  in  Leptinotarsa 
eertain  variations  or  mutations  arose  repeatedly  in  inde- 
pendent strains  and  that  by  subjecting  the  beetles  to  un- 
usual conditions  during  the  period  of  active  development 
of  their  germ  cells  the  proportion  of  these  sudden  varia- 
tions could  be  very  greatly  increased.  The  variations 
thus  produced  belonged  to  a  few  well-marked  types,  and 
while  it  would  be  hazardous  to  set  bounds  to  the  possible 
number  of  mutants  the  species  may  produce,  it  is  probable 
that  the  number  is  subjected  to  a  certain  limitation  im- 
posed by  the  peculiar  organization  of  the  germinal  sub- 
stance. 

The  selection  of  variations  by  the  germ  plasm  may  be 
illustrated  by  some  observations  of  Jennings  upon  inherit- 
ance in  Protozoa,  In  a  few  specimens  of  Paramoecium 
it  was  noticed  that  the  body  was  furnished  with  a  spine- 
like  excrescence.  During  fission  the  spine  was  trans- 
mitted to  but  one  of  the  individuals,  the  acquired  peculi- 
arity not  arising  on  the  other.  In  one  case  the  spine  was 
transmitted  through  twenty-one  generations,  when  the 


No.  509]  THE  CATEGORIES  OF  VARIATION  279 

strain  disappeared.  In  other  cases  the  spine  was 
gradually  diminished  during  successive  divisions  and 
ultimately  disappeared.  Other  anomalies  such  as  crook- 
edness, blunt  ends,  bent  tip  of  body  and  various  abnor- 
malities, provoked  by  artificial  mutilation,  while  persisting 
for  a  variable  number  of  generations,  were  eventually 
regulated  out,  leaving  a  normal  strain. 

In  like  manner  we  may  imagine  that  through  environ- 
mental changes  the  germ  plasm  becomes  affected  by 
modifications  which  in  the  course  of  a  few  generations 
become  regulated  out,  thereby  causing  a  reversion  to  the 
primitive  stock.  Reversion  may  thus  be  conceived  as  but 
a  manifestation  of  form  regulation.  Variations  to  be 
permanent  must  be  accepted  by  the  organized  structure  of 
the  germ  cells,  so  that  they  may  be  included  instead  of 
excluded  by  the  processes  of  functional  equilibration 
to  which  these  cells  like  other  parts  of  the  organism  are 
continually  subjected.  The  congruity  of  the  variation  is 
the  important  thing;  whether  the  variation  be  large  or 
small,  sudden  or  slow,  is  of  much  less  consequence. 

After  all,  it  may  be  asked,  granting  that  variations  may 
be  interpreted  in  the  manner  here  set  forth,  do  not  the 
phenomena  of  Mendelian  inheritance,  showing  as  they  do 
that  characters  may  be  separated  and  combined  in  many 
different  ways  prove  that  these  characters  must  be  borne 
by  some  sort  of  units  in  the  germ  plasm!  This  is  a  con- 
clusion which  is  adopted  by  a  large  number  of  Men- 
delians,  but,  plausible  as  it  seems,  it  is,  I  believe,  a  totally 
.erroneous  view.  In  the  first  place  it  is  open  to  question 
if  the  assumed  purity  of  the  gametes  is  a  fact  even  in 
typical  cases  of  Mendelian  inheritance,  but,  granting  that 
there  is  an  absolute  separation  of  ancestral  tendencies, 
it  by  no  means  follows  that  there  is  any  sorting  of  indi- 
vidual unit  characters  apart  from  the  complex  of  tend- 
encies which  make  for  the  production  of  the  organism 
as  a  whole.  Hereditary  anlagen  may  perhaps  be  shuffled 
and  sorted  as  wholes,  but  if  the  germ  plasm  were  com- 
posed of  discrete  parts  representing  the  unit  characters 


280  THE  AMERICAN  NATURALIST  [  VOL.  XLIII 

of  the  individuals,  the  result  would  probably  be  utter  con- 
fusion instead  of  orderly  development.  We  might  as- 
sume that  albinism  is  dependent  upon  the  peculiar  prop- 
erties of  a  single  chromosome,  that  length  of  hair  is  de- 
pendent upon  the  constitution  of  a  second  chromosome, 
that  a  short  tail  is  associated  with  a  third,  and  so  on. 
These  characters  may  not  be  represented  by  any  kind 
of  structural  element;  they  may  have  their  basis  in  the 
general  chemical  constitution  of  the  chromosome  and  be 
produced  during  development  in  a  purely  epigenetic 
fashion.  Chromosomes  probably  have  their  individual 
peculiarities  of  chemical  constitution  as  might  be  expected 
from  the  fact  that  the  chromatin  content  of  an  individual 
represents  contributions  from  many  different  ancestors. 
Each  chromosome  or  even  a  small  constituent  of  the  chro- 
mosome may  have  a  relation  to  the  inheritance  of  the 
whole  body,  but  the  peculiarities  of  one  chromatic  element 
may  dominate  in  one  part,  those  of  another  chromatic  ele- 
ment in  another.  When  albinism  is  eliminated  it  does  not 
mean  that  this  character  alone  is  separated,  but  the  anlage 
of  an  albino  organism.  Even  if  it  is  shown  that  the  num- 
ber of  separate  characters  which  Mendelize  is  greater  than 
the  number  of  chromosomes  of  the  variety,  Mendelian 
phenomena  can  be  explained  on  the  basis  of  sorting  out 
ancestral  tendencies  as  wholes  instead  of  unit  characters. 
The  facts  of  Mendelian  inheritance  at  present  known  do 
not  necessarily  give  any  support  to  the  theory  of  discrete 
bearers  of  unit  characters,  or  the  theory  of  the  inde- 
pendent variability  of  parts  as  conceived  by  De  Vries  and 
Weismann.  This  is  a  point  which,  I  believe,  needs  to  be 
emphasized  on  account  of  the  uncritical  acceptance  of 
these  views  by  so  many  writers  on  heredity  and  variation. 
The  presence  or  absence  of  certain  characters  may  be 
independent  of  the  presence  or  absence  of  certain  others, 
but  this  fact  may  very  readily  be  accounted  for  without 
having  recourse  to  a  particulate  theory  of  inheritance. 
The  mixing  up  and  separation  of  characters  in  inherit- 
ance, far  from  proving  the  independent,  variabilitv  of 


No.  509]  THE  CATEGORIES  OF  VARIATION  281 

parts,  is  just  what  renders  the  proof  of  this  theory  ex- 
ceedingly difficult.  The  alleged  independent  variability 
of  parts  is  Weismann's  strongest  proof  of  his  doctrine 
of  determinants.  If  a  pit  in  the  e.ar  or  a  white  tuft  of 
hair  on  the  head  can  be  transmitted  for  several  genera- 
tions without  involving  any  other  change  in  the  organism, 
we  are  forced  to  assume,  according  to  Weismann,  that 
there  is  a  small  part  of  the  germ  plasm  varying  independ- 
ently of  the  rest  which  forms  the  basis  or  determinant 
of  this  character.  But  the  contention  particularly  diffi- 
cult of  proof  is  that  the  characters  really  do  appear  in 
independence  of  the  other  parts  of  the  organism.  A 
variation  may  conceivably  depend  upon  a  general  change 
in  the  constitution  of  the  substance  of  heredity,  although 
manifesting  itself  most  conspicuously  in  a  single  part. 
A  pit  in  the  ear  may  be  the  most  obvious  sign  of  the  very 
slight  constitutional  differences  between  two  individuals. 
It  is  common  to  find  two  closely  allied  species  or  varieties 
differing  markedly  in  one  or  two  features  and  much  less 
conspicuously  in  numerous  other  parts  of  their  structure. 
Peculiarities  of  horns  are  sometimes  associated  with  less 
noticeable  characteristics  of  the  hair,  thus  pointing  to  a 
common  origin  of  these  features  in  some  general  modifica- 
tion of  the  ectoderm  which  in  turn  may  result  from  some 
change  affecting  the  germ  plasm  as  a  whole.  Albinism 
which  is  so  often  cited  as  a  unit  character  is  a  peculiarity 
of  far  reaching  correlations,  being  often  associated  with 
impaired  sight  or  hearing,  diminished  fertility,  and  even 
lessened  power  of  resistance  to  disease. 

To  establish  the  independent  variability  of  parts  re- 
quires a  much  closer  study  of  possible  correlatiojns  than 
has  yet  been  made.  The  task  is  rendered  particularly 
difficult  by  the  varied  combination  and  segregation  of 
ancestral  tendencies,  which  we  have  just  considered.  If 
we  can  account  for  the  independence  of  certain  characters 
on  the  ground  of  combining  and  sorting  ancestral  tend- 
encies as  wholes,  one  has  to  disprove  the  possibility  of 
applying  this  explanation  of  the  appearance  of  a  partic- 


282  THE  AMERICAN  NATURALIST  [VOL.  XLIII 

ular  variation  before  the  latter  can  be  regarded  as  giving 
evidence  of  a  corresponding  determinant.  The  burden 
of  proof  is  on  the  shoulders  of  the  upholders  of  the  doc- 
trine of  determinants  and  it  is  a  far  heavier  one  than  the 
champions  of  this  doctrine  commonly  appreciate.  Let  us 
suppose  that  among  the  various  sets  of  hereditary  tend- 
encies that  find  expression  in  the  organization  of  an  indi- 
vidual one  should  include  the  production  of  a  particular 
variation  in  a  single  small  part.  The  constitutional  differ- 
ences which  may  go  along  with  this  peculiarity  and  of 
which  it  may  be  regarded  as  one  expression  may  be  modi- 
fied or  kept  from  becoming  manifest  by  other  and  rival 
sets  of  hereditary  tendencies  thereby  rendering  it  almost 
impossible  to  detect  the  correlations  that  really  exist,  and 
giving  the  character  the  delusive  appearance  of  independ- 
ence. The  question  of  the  independent  variability  of 
parts  is  a  crucial  one  for  the  particulate  theories  of  in- 
heritance, but  it  is  one.  so  beset  with  practical  difficulties 
that  a  final  answer  may  not  soon  be  forthcoming. 

In  the  preceding  discussion  the  attempt  has  been  made 
to  show  that  the  various  categories  of  variations  recog- 
nized by  De  Vries  and  others  are  not  sharply  separable 
either  on  morphological  grounds  or  by  their  behavior 
when  subjected  to  crossing  experiments.  The  attempt 
was  made  also  to  show  that  neither  the  facts  of  variability 
nor  those  of  Mendelian  inheritance  give  any  support  to 
the  doctrine  of  pangens,  determinants,  or  other  assumed 
bearers  of  unit  characters,  and  that  unit  characters,  as 
elements  than  can  enter  or  depart  from  the  complex  of 
tendencies  that  make  up  an  organism  probably  have  no 
existence.  It  is  evident  that  variations  differ  in  their 
stability,  but  the  explanation  of  this  fact  may  lie  in  the 
physiological  relations  of  the  variation  rather  than  in 
some  hypothetical  representative  unit.  Whether  the 
variations  of  the  discontinuous  type  have  been  influential, 
in  any  marked  degree,  in  shaping  the  course  of  evolution 
is  a  question  upon  which  we  need  much  more  evidence. 
Mutations,  as  we  have  seen,  may  be  very  small  affairs. 


No.  509]  THE  CATEGOEIES  OF  VARIATION  283 

About  the  only  criterion  by  which  they  may  be  recognized 
is  their  stability,  and  even  that  gives  some  evidence  of 
being  a  matter  of  degree.  No  limit  has  been  discovered 
to  the  minuteness  of  the  stable  modifications  that  may 
occur,  and  it  may  happen  that  further  study  will  reveal 
the  comparatively  frequent  appearance  of  very  slight 
variations  of  this  kind.  In  fact,  considerable  progress 
has  even  now  been  made  in  this  direction  by  the  study  of 
grains ;  and  the  number  of  more  or  less  stable  modifica- 
tions that  are  likely  to  be  discovered  threatens  to  over- 
whelm systematists  with  the  labor  of  naming  and  describ- 
ing them.  In  many  organisms  not  propagated  by  self- 
fertilization  the  detection  of  these  small  steps  is  no  easy 
task  and  the  attempt  to  describe  them  all  would  undoubt- 
edly prove  a  futile  effort.  Among  human  beings,  for 
instance,  what  are  we  to  designate  as  elementary  species  1 
We  meet  with  all  grades  of  differences  from  well-marked 
family  traits  to  those  which  separate  the  Caucasian  from 
the  negro.  Are  we  to  regard  the  Hapsburg  lip  which 
was  transmitted  with  fidelity  for  many  generations  as 
the  mark  of  an  elementary  species'?  It  was  apparently 
a  new  character  and  therefore  presumably  dependent 
upon  a  new  pangen  or  determinant.  The  Celts,  Teutons, 
Slavs,  etc.,  differ  by  more  or  less  constant  characters 
which  are  constitutional  and  not  confined  to  single  parts, 
and  the  same  may  be  said  of  the  various  subdivisions  of 
these  groups.  The  Aryan  stock  to  which  these  groups 
belong  is  separated  by  still  greater  differences  from  the 
other  subdivisions  of  the  Caucasian  race,  and  the  latter 
in  turn  differs  still  more  widely  from  the  negroes  and 
Mongolians.  One  has  considerable  difficulty  in  disposing 
of  these  groups  either  as  varieties  or  elementary  species. 
They  can  not  from  De  Vries  standpoint  be  considered 
the  results  of  fluctuating  variability  on  account  of  their 
constancy  even  under  very  varied  external  conditions. 
If  the  small  divisions  have  arisen  by  slow  changes,  as 
everything  indicates,  there  is  no  logical  halting  place 
short  of  admitting  that  the  greater  ones  may  have  done 


284  THE  AMERICAN  NATURALIST          [VOL.  XLIII 

the  same.  In  fact  a  survey  of  the  racial  differences  of 
man  in  their  varying  degrees  and  kinds  and  their  correla- 
tion with  geographical  distribution  shows  us  pretty 
clearly  that  these  differences  have  been  slowly  acquired 
by  the  summation  of  very  small  variations.  These  groups 
are  not  related  as  the  so-called  retrograde  or  digressive 
varieties  are,  but  they  are  based  on  differences  in  general 
constitution  affecting  the  shape  of  the  skull,  the  character- 
istic complexion,  the  general  temperament,  and  many 
other  traits  too  numerous  to  specify.  They  may  have 
arisen  by  minute,  discrete,  stable  variations,  but  to  call 
each  step  in  advance  an  elementary  species  seems  absurd, 
and  to  talk  of  the  immutability  of  species  still  more  so. 
We  gain  little  by  characterizing  as  elementary  species 
the  small  steps  of  which  there  may  be  a  dozen  or  more 
separating  a  German  from  a  Frenchman. 

Students  of  geographical  distribution  as  a  rule  set 
little  store  by  the  theory  of  mutation.  The  relation  of 
variation  and  species-forming  to  distribution  as  illus- 
trated by  the  work  of  Gulick  and  Hyatt  on  the  Achatin- 
nelidae,  the  Sarasins  on  the  snail  fauna  of  Celebes,  of 
Plate  on  the  mollusca  of  the  Bahamas,  and  of  many  stu- 
dents of  the  mammals,  birds  and  fishes  of  North  America 
indicate  that  the  steps  concerned  in  species-forming  have 
been  very  modest  ones.  If  sudden  mutations  of  consid- 
erable magnitude  have  been  a  not  uncommon  source  of 
varieties  of  domesticated  animals  and  cultivated  plants 
it  does  not  follow  that  the  selection  of  comparatively 
small  variations  has  not  been  the  predominant  method 
of  species-forming  in  a  state  of  nature. 

After  fifty  years  from  the  publication  of  Darwin's 
"Origin  of  Species"  we  are  still  debating,  and  more 
lively  than  ever,  the  central  problem  of  that  epoch-making 
book;  but  it  is  not  improbable  the  views  of  its  sagacious 
author  will  prove  more  nearly  correct  than  those  of  most 
of  his  modern  critics.  Much  remains  to  be  done  before 
the  problem  is  finally  solved,  and  there  are  few  fields 


No.  509]  THE  CATEGORIES  OF  VARIATION  285 

before    the    investigator    that    are    more    fruitful    and 
alluring.3 

3  Mendelism  and  Unit  Characters. — Since  this  article  on  the  Cate- 
gories of  Variation  was  sent  in  for  publication  several  articles  have 
appeared  whose  contents  would  have  been  referred  to  had  they  been  pub- 
lished somewhat  earlier.  One  of  these  is  a  short  paper  by  W.  J.  Spillman  in 
"The  Nature  of  Unit  Characters/'  in  the  April  number  of  this  journal,  in 
which  an  interpretation  of  so-called  unit  characters  is  given  which  is,  in 
many  respects,  similar  to  my  own.  Reference  to  Mr.  Spillman 's  views  would 
naturally  be  expected  in  an  article  appearing  later  than  his  in  the  same 
journal,  so  that  it  may  be  well  to  state  that  the  proof  of  my  paper  was 
returned  to  the  publishers  in  the  first  part  of  February,  and  that  no 
modification  of  the  paper  has  since  been  made.  I  am  glad  to  find  myself  in 
agreement  with  Mr.  Spillman  at  least  to  the  extent  that  the  facts  of 
Mendelian  inheritance  do  not  compel  us  to  adopt  a  particulate  theory  of 
heredity.  That  Mendelian  inheritance  can  be  explained  by  the  sorting 
process  which  is  supposed  to  take  place  in  the  reducing  divisions  of  the  germ 
cells  I  feel  by  no  means  assured,- there  are  grave  difficulties  in  the  way  of 
such  an  interpretation.  But  if  this  explanation  prove  to  be  the  correct  one 
it  would  be  far  from  justifying  the  commonly  accepted  doctrine  of  unit 
characters  with  all  its  evolutionary  implications. 

Reference  might  also  be  made  to  some  recent  articles  by  De.Vries,  espe- 
cially one  on  the  crosses  of  Oenothera  nanella  (Ueber  die^Kwillingsbas- 
tarde  von  Oenotliera  nanella.  Ber.  Bot.  Ges.  26,  p.  667,  '08),  inasmuch  as 
the  experiments  reported  tend  to  strengthen  further  the  contention  of  the 
present  writer  that  no  sharp  line  can  be  drawn  from  the  results  of  crossing 
experiments  between  elementary  species  and  so-called  retrograde  varieties. 


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